This invention relates to speed sensors and particularly to speed sensors of the flyweight type utilized in a turbine type of power plant's fuel control.
As is generally well known in technology encompassing fuel controls for turbine types of power plants, the fuel controls such as the JFC-12, JFC-25 and JFC-60 manufactured by the Hamilton Standard Division of the assignee and the type exemplified in U.S. Pat. No. 2,822,666 granted to S. G. Best on Feb. 11, 1958 and also assigned to the same assignee are designed on the W.sub.f/P .times. P principal,
Where W.sub.f = fuel flow in pound/hr. PA1 P = compressor discharge pressure
This type of control which can be considered as having a logic network or computer section and a metering section monitors certain engine operating parameters such that during steady-state W.sub.f/P is made a function of compressor speed (N) and during acceleration W.sub.f/P is made a function of compressor speed and compressor inlet temperature (T.sub.T2). Thus, the fuel control serves to schedule fuel flow so as to achieve the desired engine speed while assuring that the flow of fuel does not permit surge, overheating, rich or lean blowout.
Under certain circumstances, one of the requirements of the fuel control is to provide for overspeed protection which may be needed during an emergency situation, such as where the flyweight speed sensor drive shaft malfunctions. In this event, the computer section positions the control linkage to an adjustable stop, generally known as the low speed saturation point which serves as the point where the speed servo provides the emergency schedule, based on a function of temperature manifested by the existing acceleration cam.
The problem encountered in certain fuel controls, however, is that the point at which the transition from the normal schedule to the emergency schedule occurs has been indiscriminate, inasmuch as the point at which the emergency schedule was activated or deactivated occurred over a wide variation of low speed saturations and that the speed of this transition was not repeatable. For purposes of starting the engine at a predetermined speed and prevention of overspeed at the low end of the speed spectrum, it is abundantly important that a particular speed at this transitory point and the repeatability of this speed be obtained.
By discovering the source of the problem which not only prevented the heretofore known fuel control to achieve the transitory point at a predetermined speed but repeating the speed at which the transitory point occurred we were able to obviate this problem in existing fuel controls. Thus, we found that a torsional spring applying a force to the flyweights at the low end of the speed spectrum served to define the transitory point so that it always occurred at substantially the same speed. By virtue of the torsional spring the loads applied to each face of the flyweights summed at its toes produce a force greater than the flyweight force at the low speed saturation condition, thus insuring that the thrust bearing is always preloaded and could not shift occasioned heretofore by the moment on the bearing caused by the feedback spring. Additionally, the torsion spring allows use of existing hardware with slight modifications and fits into the existing envelope which is not so for conventional leaf springs, or compression springs that require seats or method of attachment extraneous to useful functional hardware.